Receiving section of a telephone

ABSTRACT

A processing unit with balanced outputs transfers a received digital signal to an amplification unit with balanced inputs and outputs. A control unit enables or disables the processing and amplification units in response to a power up/power down signal. To prevent disturbances due to power up/power down transients from appearing in a speaker connected between the outputs of the amplification unit, switches are provided between the outputs of the processing unit and the inputs of the amplification unit. A delay circuit generates according to a predetermined timing program enabling/disabling control signals for the processing and amplification units, and generates control signals for the switches.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of transceivers, and, more particularly, to a receiver section of a telephone.

BACKGROUND OF THE INVENTION

[0002] A signal received by a telephone from a digital telephone network is demodulated, processed in digital form, converted to analog form, and is amplified before being applied to an electroacoustic transducer. The receiver section of a telephone can be represented schematically as shown in FIG. 1.

[0003] A demodulated digital signal RX-IN produced by a receiving and demodulating unit 10 is applied to the input of a processing unit 11. The processing unit 11 converts the demodulated digital signal to analog form, filters and amplifies the analog signal. The analog signal leaving the processing unit 11 is applied to the input of a power amplifier 12 for transferring the signal to an electroacoustic transducer 13, such as an electromagnetic earphone or speaker, with sufficient electrical power for its operation.

[0004] The gain of the power amplifier 12 is determined by the ratio between the resistance of a feedback resistor R2 connected between the output and the inverting input of the amplifier, and by the resistance of a resistor R1 connected between the output of the processing unit 11 and the inverting input of the amplifier.

[0005] To prevent the dissipation of power when the telephone is not in use, circuit means are provided both in the processing unit 11 and in the amplifier 12 to respond to an external enabling or disabling command. This function is initiated via a terminal connection of the processing unit 11 and via a terminal connection of the amplifier 12 in response to a digital power down PD signal. The power down PD signal may be a single bit signal. The receiver section is put into a state of zero current absorption when PD=1 (power down), and is enabled to absorb power supply current when PD=0 (power up).

[0006] When a change is made from the power down state to the power up state, or vice versa, both the processing unit 11 and the power amplifier 12 are subjected to abrupt voltage and current transients before returning to a normal operating condition. During these transients, electrical disturbances are produced by components having frequencies in the acoustic band, which results in an audible noise in the speaker 13 that may be annoying.

[0007] Various arrangements requiring the addition of active or passive components in series and/or in parallel with the speaker 13 for filtering undesired electrical disturbances have been adopted to overcome this problem. This approach, however, requires relatively bulky components outside the power amplifier which may normally be formed as an integrated circuit.

SUMMARY OF THE INVENTION

[0008] An object of the present invention is to provide a receiving section for a telephone which is not subject to the problem described above and which can easily be integrated.

[0009] This object is achieved by making the receiving section defined and characterized in a general way in the first claim.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will be more clearly understood from the following detailed description of various embodiments, provided by way of examples, and therefore, without any restrictions, with reference to the attached drawings, in which:

[0011]FIG. 1 is a block diagram of a receiver section of a telephone according to the prior art; and

[0012] FIGS. 2-4 are block diagrams respectively illustrating an embodiment according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] In FIG. 2, elements identical or corresponding to those of FIG. 1 are indicated by the same reference numbers. A processing unit 11 with a balanced structure receives at its input a demodulated digital signal RX-IN, and supplies a balanced analog signal at two outputs. The outputs of the processing unit 11 are coupled to the inputs of a differential amplifier 12. More particularly, each output is connected to one of the inputs of the amplifier through a respective resistor R1A, R1B in series with an n-channel MOS transistor M1A, M1B operating as an electrical switch.

[0014] Each of the differential outputs 15, 16 of the amplifier 12 is connected, respectively, to the inverting and non-inverting inputs of the amplifier through a resistor R2A, R2B in series with a MOS transistor M2A, M2B. Each MOS transistor M2A, M2B is biased for conduction. An electroacoustic transducer 13, i.e., a speaker, is connected between the outputs 15, 16 of the differential amplifier 12.

[0015] The processing unit 11 and the differential amplifier 12 are supplied power from a voltage source. The processing unit 11 and the differential amplifier 12 each have circuit means which enables or disables the power supply according to a signal applied to respective enabling terminals 17, 18.

[0016] A control unit 20 includes a delay circuit 24 and a logic circuit 21, and has an input terminal 19 to which is applied a binary power down PD signal generated by a central circuit control unit (not shown) of the telephone. The logic circuit 21 responds to the power down PD signal by causing the closing or opening of an electrical switch 22 and the closing or opening of an electrical changeover switch 23. The switch 22 connects the terminal 19 to the enabling terminal 17 of the processing unit 11. The changeover switch 23 selectively connects the terminal 19 to the enabling terminal 18 of the differential amplifier 12 directly or through the delay circuit 24.

[0017] The changeover switch 23 is used to additionally apply the power down PD signal, directly or through the delay circuit 24, to the gate electrodes of the n-channel MOS transistors M1A and M1B through an inverter 25. The inverter is necessary because the n-channel transistors M1A and M1B must be closed when the amplifier 12 is activated. Since this happens when the binary power down PD signal is at a low level (PD=0), this signal must be inverted so that it has the necessary positive voltage to operate the n-channel transistors M1A and M1B. If p-channel transistors were used in place of the n-channel transistors, the inverter 25 would not be necessary.

[0018] The operation of the circuit will now be considered. The delay circuit 24, the switch 22, the changeover switch 23 and the logic circuit 21 form, in combination, a delay unit which produces control signals according to a predetermined timing program. In particular, the logic circuit 21 produces a signal Y=1 if the power down PD signal is changing from 1 to 0, i.e., from a power down state to a power up state. The logic circuit 21 produces a signal Y=0 if the power down PD signal is changing from the 0 to 1, i.e., from a power up state to a power down state.

[0019] When Y=1, the position of the switch 22 and that of the changeover switch 23 are as shown in the drawings. In other words, a 0 signal is applied to the terminal 17 at the instant of the closing of the switch 22, and the 0 signal is applied to the terminal 18 after a period of delay fixed by the delay circuit 24. The delay period is chosen to be sufficiently long to allow the processing unit 11 to settle. Thus the disturbances present at the output of the processing unit 11 have no effect on the amplifier 12 or on the speaker 13 because the amplifier remains off and the transistors M1A and M1B act as open circuits during the settling of the processing unit.

[0020] At the end of the delay period fixed by the delay circuit 24, the amplifier 12 is also activated and the outputs of the processing unit 11 are connected to the inputs of the amplifier through the MOS transistors M1A and M1B when they are conducting. Since the input signal of the differential amplifier 12 is stable, any disturbances at the output of the amplifier is limited to the settling of the amplifier 12.

[0021] However, since the structure of the amplifier 12 is balanced, the voltages present at the two outputs 15, 16 due to any settling disturbances will be in phase with each other, and therefore, will not have any effect on the differential output signal Vout. Consequently, there will not be any disturbances in the speaker 13.

[0022] In the presence of a power down signal (PD from 0 to 1), the signal emitted by the logic circuit 21 is at low level (Y=0). This causes the opening of the switch 22 and the switching of the changeover switch 23. Therefore, the amplifier 12 is disabled, the switches formed by the transistors M1A and M1B are opened and the processing unit 11 is disabled.

[0023] The settling of the amplifier 12 has no effect on the output since the structure is a differential amplifier. The processing unit 11 has no effect on the output of the receiver section because the connections between the processing unit 11 and the amplifier 12 are interrupted by the opening of the transistors M1A and M1B. The disabling of the amplifier 12 takes place simultaneously with the disabling of the processing unit 11 to avoid any disturbances in the speaker 13.

[0024] The function of the transistors M2A and M2B biased for conduction is to introduce a supplementary resistance into the feedback circuit for the determination of the gain. The transistors M2A and M2B are biased for conduction. The supplementary resistance is equal to that of the transistors M1A and M1B when conducting. The gain of the amplifier 12 is determined from the ratio between the feedback resistance and the resistance in series with the input terminal.

[0025] In the design of an integrated circuit, it is not possible to precisely specify the resistance of a single resistor due to the wide variations in the manufacturing parameters, as readily understood by one skilled in the art. However, it is possible to produce two resistors having a precise ratio between their resistances.

[0026] The insertion of a respective transistor M1A, M1B in series with each of the inputs, and consequently with the respective resistors, modifies the ratio fixed at the design stage between the feedback resistance and the resistance in series with the inputs. The resistance of each of the conducting transistors M2A, M2B in series with the feedback resistors compensates for this modification of the ratio.

[0027]FIG. 3 shows a variantion in which the controlled connection between the processing unit 11 and the amplifier 12 is by electrical switches M1A′, M1B′ connected before the resistors R1A and R1B instead of after them as shown in FIG. 2. In this case, it is preferable for the switch to be formed by pairs of complementary transistors since the dynamic range of the signal is greater than that of the circuit shown in FIG. 2. To compensate the supplementary resistances of the pairs of complementary transistors, pairs of identical transistors M2A′ and M2B′ are provided in series with the feedback resistors R2A, R2B, and are preferably positioned after them.

[0028]FIG. 4 shows another variantion in which the processing unit 11 and the amplifier 12 have their corresponding enabling/disabling terminals 17, 18 both connected to the switch 22 so that they are enabled or disabled simultaneously. This may be done while the control terminals of the transistors M1A, M1B are connected through the inverter 25 to the changeover switch 23 as shown in FIG. 2.

[0029] With this configuration, the power-up time of the receiver section is shorter than that of the embodiment shown in FIG. 2. This is so since the power-up time is not the sum of the power-up time of the processing unit 11, the delay period of the circuit 24, and that of the amplifier 12. Instead, the power-up time is determined by the delay period of the delay circuit 24, i.e., by the settling time of the processing unit 11.

[0030] The receiving section according to the present invention makes it possible to avoid disturbance in the speaker 13 since the propagation of the signal during the settling transients is substantially blocked. This is achieved without bulky external components, and is accomplished by using a few supplementary components which can easily be integrated. 

That which is claimed is:
 1. Receiving section of a telephone comprising a unit (10) for receiving and demodulating the signal, a unit (11) for processing the demodulated signal, having two balanced outputs and having first controllable enabling/disabling means (17), an amplification unit (12) having two balanced inputs, each coupled to one of the outputs of the processing unit (11), having two balanced outputs (15, 16) coupled to an electroacoustic transducer (13) and having second controllable enabling/disabling means (18) and a control unit (20) which is connected to the first and to the second enabling/disabling means (17, 18) of the processing and amplification units (11, 12) and which is capable of producing enabling/disabling control signals in response to a power up/power down signal (PD), characterized in that it comprises controllable switch means (M1A, M1B) connected between the outputs of the processing unit (11) and the inputs of the amplification unit (12) and in that the control unit (20) comprises delay means (21-24) for producing the control signals of the first and second enabling/disabling means (17, 18), and for producing control signals for the switch means (M1A, M1B) according to a predetermined time program.
 2. Receiving section according to claim 1, in which the delay means (21-24) respond to a power up signal by producing an enabling control signal for the first and second enabling/disabling means (17, 18) of the processing unit (11) and of the amplification unit (12) and, with a predetermined delay (24) with respect to the power up command, a closing control signal for the switch means (M1A, M1B), and respond to a power down signal by producing a disabling signal for the first and second enabling/disabling means (17, 18) of the processing unit (11) and of the amplification unit (12).
 3. Receiving section according to claim 2, in which the delay means (21-24) respond to the power down signal with a signal for opening the switch means (M1A, M1B), this signal being approximately synchronous with the disabling signal.
 4. Receiving section according to claim 1, in which the delay means (21-24) respond to a power up signal by producing a first enabling control signal for the first enabling/disabling means (17) of the processing unit (11) and, with a predetermined delay (24) with respect to the power up control signal, a second enabling control signal for the second enabling/disabling means (18) of the amplification unit (12) and a closing control signal for the switch means (M1A, M1B), and respond to a power down signal by producing a disabling signal for the first and the second enabling/disabling means (17, 18) of the processing unit (11) and of the amplification unit (12).
 5. Receiving section according to claim 4, in which the delay means (21-24) respond to the power down signal with a signal for opening the switch means (M1A, M1B), this signal being approximately synchronous with the disabling signal.
 6. Receiving section according to any one of the preceding claims, in which the amplification unit comprises a differential amplifier (12) with two inputs and two outputs and means of determining the gain, comprising first resistive means (R2A, R2B, M2A, M2B) connected between inputs and outputs and second resistive means (R1A, R1B) connected in series with the inputs, each of the first resistive means comprising a resistor (R2A, R2B) and a closed switch (M2A, M2B) having the same resistance as the corresponding switch means (M1A, M1B) when in the closed state.
 7. Receiving section according to claim 6, in which the switch means (M1A, M1B) are inserted between the inputs of the differential amplifier (12) and the corresponding second resistive means (R1A, R1B) and the closed switches (M2A, M2B) are inserted between the corresponding resistors (R2A, R2B) and the corresponding inputs of the differential amplifier (12). 